1. Field of the Invention
This invention relates to infrared fibers and more particularly, it is concerned with infrared transmitting fibers consisting of mixed crystals of silver bromide and silver chloride.
2. Description of the Prior Art
Various efforts have hitherto been made to prepare infrared fibers by extruding infrared transmitting optical crystals or by growing fibrous single crystals ("Nikkei Electronics" 1980, Dec. 8, page 140). There have been proposed infrared fibers each comprising crystalline fiber 1 as a core inserted loosely in tubular protective layer 3 to retain gap 2 as a cladding as shown in FIG. 1, and infrared fibers of step index type as shown in FIG. 2 and FIG. 3.
Silver chloride and silver bromide are suitable for use as an infrared transmitting material because of having a wide transmission range and being capable of transmitting CO.sub.2 laser beam (wavelength 10.6 .mu.m) with a small loss.
Infrared fibers of silver halide crystals are known as a step index fiber of silver chloride clad silver bromide, which can be prepared by extruding silver halide optical crystals as described in Japanese Patent Application OPI (Kokai) No. 121406/1980 and U.S. Pat. No. 4253731. However, when an infrared fiber is made of such a combination of silver chloride and silver bromide, the disadvantages are unavoidable that its tensile strength and yield strength are small and when the fiber is extruded or bent it tends to be scratched or broken. This tendency is particularly remarkable when impurities are removed from the fiber materials so as to decrease the light transmission loss.
On the other hand, Japanese Patent Application OPI (Kokai) No. 124103/1980 describes an infrared fiber with a core consisting of silver bromide or a melt mixture of silver bromide and silver chloride and a cladding consisting of a melt mixture of silver bromide in a proportion less than in the core and silver chloride or silver chloride. In particular, the composition of the core comprising at most 50 mol % of silver bromide and the balance silver chloride, e.g. 5-30 mol % of silver bromide and the balance silver chloride is preferred from the standpoint of transmission characteristics. In this case, however, the tensile strength and yield strength are increased, but the elongation is decreased to a great extent and the fiber is hard and brittle and tends to be broken by operations such as bending.